1. Field of the Invention
The present invention relates to a bivalent metal silicate phosphor which emits a blue color under excitation by ultraviolet rays (UV) or vacuum ultraviolet rays (VUV) having a wavelength of at most 200 nm, and a process for its production, as well as a phosphor paste composition containing such a phosphor, a vacuum ultraviolet ray excitation type light-emitting device (VUV excitation type light-emitting device) and a fluorescent lamp.
2. Discussion of Background
In recent years, there have been various research and development activities on a VUV excitation type light-emitting device of a structure in which a rare gas such as Ar, Xe, He, Ne or a gas mixture thereof, is sealed in an envelop formed of e.g. glass, and a phosphor layer made of a phosphor for VUV formed inside of the envelop, is excited by VUV generated by discharge of such a rare gas, to emit light, as represented by e.g. a plasma display panel (PDP) or a rare gas lamp to be used, for example, as a light source for reading by a scanner.
In a rare gas lamp as a typical example of such a VUV excitation type light-emitting device, a rare gas such as Xe or Xe-Ne is sealed in a glass tube, and on the inner wall surface of such a tube, a phosphor layer made of a phosphor for VUV, which emits light when excited by VUV, is formed. When an electrical energy is applied between electrodes of such a rare gas lamp, discharge of the rare gas takes place in such a glass tube, and the phosphor layer formed on the inner wall surface of the tube, will be excited by VUV thereby generated, to emit visible light.
Whereas, PDP as another typical example of the VUV excitation type light-emitting device, can be in principle regarded to be one wherein the above-mentioned VUV excitation type rare gas lamp is further downsized, and such downsized rare gas lamps of different three colors are arranged in stripes or matrices. Namely, it is one having restricted discharge spaces (cells) arranged in stripes or matrices. Each cell is provided with electrodes, and a phosphor layer made of a phosphor for VUV, is formed inside of each cell. A rare gas such as Xe, Xe-Ne, He-Xe or He-Ne-Xe, is sealed in each cell, so that when an electrical energy is applied from the electrodes in the cell, discharge of the rare gas takes place in the cell to generate VUV, whereupon the phosphor layer in the cell will be excited by this VUV to emit visible light, and by this emitted light, an image will be displayed. In a case of full color PDP, cells having phosphor layers made of phosphors which emit red, blue and green, respectively, under VUV excitation, are arranged in stripes or matrices, whereby full color display can be carried out.
As the phosphors for forming phosphor layers for such VUV excitation type light-emitting devices, red-emitting phosphors such as (Y,Gd)BO3:Eu, green-emitting phosphors such as LaPO4:Ce,Tb, (Ba,Sr)MgAl10O17:Eu,Mn and Zn2SiO4:Mn, and blue-emitting phosphors such as BaMgAl10O17:Eu, are, for example, used alone or in combination as a mixture depending upon the desired color for emission (see Journal of Electronic Material, December 1997 issue, Kogyo Chousa K.K., etc.). Among such phosphors for VUV which are practically used for phosphor layers of VUV excitation type light-emitting devices, a phosphor which is practically used mainly as a blue-emitting component, is an aluminate phosphor so-called BAM having a composition of BaMgAl10O17:Eu. This BAM phosphor has a high luminance brightness when excited under irradiation by VUV, and the color purity as a blue color is excellent. However, it has drawbacks such that the deterioration of luminance in the baking step (the deterioration by baking) in the formation of the phosphor layer for a VUV excitation type light-emitting device employing this phosphor, is substantial, and the deterioration with time of the luminance brightness when exposed to VUV for a long period of time by driving the VUV excitation type light-emitting device (deterioration by VUV) is substantial.
Accordingly, it is desired to develop a blue-emitting VUV excitation type phosphor which is less susceptible to deterioration by baking or by VUV. As a proposal to overcome such problems, a bivalent metal silicate phosphor containing Eu as an activator and having a composition represented by the formula CaMgSi2O6:Eu, has been reported as one of blue-emitting phosphors which are less susceptible to deterioration by baking and by VUV (see Proceedings of The 8th International Display Workshops 2001, pp. 1115). However, this phosphor has a problem that the luminance is low as compared with BAM being a conventional blue-emitting phosphor, and a study is being made to improve the luminance to a practical level.
Further, although this phosphor is said to be less susceptible to deterioration by baking or by VUV, it is not necessarily practically adequate especially with respect to deterioration by VUV, and a further improvement has been desired.
Further, with respect to the process for producing a phosphor, which is substantially influential over such a quality, it is disclosed as common to use, for the phosphor in question, CaCO3 as a material for Ca, MgCO3 or 3MgCO3·Mg(OH)2 as a material for Mg, SiO2 as a material for Si and Eu2O3 as a material for Eu. On the other hand, it is known that even if these materials are blended and baked, it is impossible to form a bivalent metal silicate phosphor having a composition represented by the formula CaMgSi2O6:Eu, which has an adequate emission intensity as a blue-emitting phosphor.
Whereas, as another method for producing such a bivalent metal silicate phosphor, it has been proposed to use EuF3 instead of Eu2O3, and it is reported that a phosphor showing a relatively strong blue emission with high color purity, can be obtained (see Proceedings of The 8th International Display Workshops 2001, pp. 1115).
However, in order to form a sufficiently uniform and dense phosphor layer to be practically free from any problem for a VUV excitation type light-emitting device such as a rare gas lamp or PDP, it is necessary to let phosphor particles have proper powder characteristics. Specifically, the particle diameter D50 as measured by Coulter Counter Method is required to be at most 10 μm, preferably from about 1 to 7 μm, more preferably from about 1 to 4 μm, and further, with respect to the particle size distribution, σ log(L) and σ log(S) are desired to be at most 0.5. In a conventional process of employing EuF3, the particle size of the phosphor particles thereby obtainable, tends to be too large, and it has been impossible to obtain phosphor particles having powder characteristics within a proper range to form a phosphor layer as mentioned above.
Further, with PDP of AC type, it is known that the discharge initiation voltage will be influenced and will change by the electrification tendency of the coated phosphor. For example, with BAM or (Y,GD)BO3:Eu which tends to be positively electrified, the discharge initiation voltage tends to be low, while with Zn2SiO4:Mn which tends to be negatively electrified, the discharge initiation voltage tends to be high. From the aspect of a circuit, the lower the discharge initiation voltage, the better. CaMgSi2O6:Eu prepared by a conventional method, tends to be negatively electrified and thus requires a high voltage for the initiation of discharge. This is also one factor that CaMgSi2O6:Eu is not practically used for PDP. Here, the electrification tendency can be evaluated by measuring the blow off electrostatic charge of the material in question. Specifically, a phosphor powder and poval resin beads are mixed and shaked to let them undergo triboelectrification, whereby the blow off electrostatic charge is measured to evaluate the electrostatic charge of the phosphor powder.